Start with a sketch
Draw the room from above and mark doors, windows, alcoves and permanent obstacles. Use a tape measure or a suitable laser measure. Write every measurement on the sketch so you do not confuse wall length with wall height.
For a rectangular floor
Measure length and width in feet. Multiply them to get square feet. A 12-foot by 10-foot room has 120 square feet of floor. For an L-shaped room, split the floor into two rectangles that do not overlap, then add their areas. Do not use the outer bounding rectangle if it includes space outside the room.
For painting walls
Measure the room perimeter and multiply by wall height. In a rectangular room, perimeter is twice the sum of length and width. Measure doors and windows separately and subtract their combined area when they will not be painted. For sloped ceilings or walls of different heights, measure each wall independently.
Convert feet and inches correctly
Divide inches by 12 and add the result to the feet. For 12 feet 6 inches, enter 12.5. For 10 feet 3 inches, enter 10.25. Do not enter 12.6 for 12 feet 6 inches: that represents a different length.
Measure twice before buying
Repeat the measurements and compare them with your sketch. Keep the original dimensions separate from your waste allowance; adding waste twice inflates the order. Use the product label to determine coverage per gallon or box.
Calculate paint gallons or calculate flooring boxes with your measurements.